I don't have a firm answer. I'd wouldn't be surprised if the stored procedure
was doing a table scan due to the work that occurs in the WHERE clause

>    (CAST (CAST (t.nMONTH AS VARCHAR)
>    + '/01/' + CAST (t.nYear AS VARCHAR)
>    AS DATETIME)
>    >= CAST (CAST(@M3 AS VARCHAR)+'/01/'+CAST(@Y3 AS VARCHAR) AS DATETIME) AND
>    CAST (CAST (t.nMONTH AS VARCHAR)
>    + '/01/' + CAST (t.nYear AS VARCHAR)
>    AS DATETIME)
>    < CAST (CAST(@M4 AS VARCHAR)+'/01/'+CAST(@Y1 AS VARCHAR) AS DATETIME))  and
>	t.CDISTmgr = @cDistMgr

I wonder if the query optimizer is doing somekind of substitution on these values when the query is submitted as ad-hoc.

Does the stored procedure speed up if you create the DATETIME values outside of the query? At least two of them appear to be constant once the variables are assigned.

-Mike